Zeolitic materials, both natural and synthetic, have been demonstrated to have catalytic properties for various types of hydrocarbon conversion. It is often advantageous to dealuminate these materials in order to improve their process performance. Performance measures include product selectivity, product quality and catalyst stability. Conventional techniques for zeolite dealumination include hydrothermal treatment, mineral acid treatment with HCl, HNO.sub.3, and H.sub.2 SO.sub.4, and chemical treatment with SiCl.sub.4 or EDTA. The treatments, however, do not exhibit selectivity to the zeolite crystal surface.
U.S. Pat. No. 3,442,795 to Kerr et al. describes a process for preparing highly siliceous zeolite-type materials from crystalline aluminosilicates by means of a solvolysis, e.g. hydrolysis, followed by a chelation. In this process, the acid form of a zeolite is subjected to hydrolysis, to remove aluminum from the aluminosilicate. The aluminum can then be physically separated from the aluminosilicate by the use of complexing or chelating agents such as ethylenediaminetetraacetic acid or carboxylic acid, to form aluminum complexes that are readily removable from the aluminosilicate. The examples are directed to the use of EDTA to remove alumina.
EP 0 259 526 B1 discloses the use of dealumination in producing ECR-17. The preferred dealumination method involves a combination of steam treatment and acid leaching, or chemical treatments with silicon halides. The acid used is preferably a mineral acid, such as HCl, HNO.sub.3 or H.sub.2 SO.sub.4, but may also be weaker acids such as formic, acetic, citric, oxalic, tartaric acids and the like.
U.S. Pat. No. 4,388,177 discloses modifying the shape selectivity of natural ferrierite by treating with oxalic acid to impart catalytic activity.
U.S. Pat. No. 4,088,605 discloses a crystalline aluminosilicate zeolite containing an aluminum-free outer shell prepared by initiating the crystallization in a crystallization medium and then altering the crystallization medium to eliminate the aluminum therein. This can be accomplished by a total replacement of the reaction mixture or by complexing from the original reaction mixture any remaining aluminum ion with reagents such as gluconic acid, tartaric acid, nitrilotriacetic acid or EDTA.
Non-selective reactions on the surface acid sites of the zeolite are generally undesirable. These non-selective reactions often lead to lower product yield and/or inferior product characteristics. To minimize the incidence of undesirable reactions occurring on the surface of the zeolite catalyst methods have been used to reduce or eliminate surface acidity by extraction with bulky reagents or by surface poisoning.
Zeolite modification by exchange and similar technology with large cations such as N.sup.+ and P.sup.+ and large branched compounds such as polyamines and the like is described in U.S. Pat. No. 4,101,595. Bulky phenolic and silicating zeolite surface modifying agents are described in U.S. Pat. Nos. 4,100,215 and 4,002,697, respectively. The surface acidity of the zeolite can be eliminated or reduced by treatment with bulky dialkylamine reagents as described in U.S. Pat. Nos. 4,520,221 and 4,568,786.
U.S Pat. No. 4,716,135 discloses zeolite catalysts can be surface inactivated by cofeeding a sterically hindered base organophosphorus compound. U.S. Pat. No. 5,080,878 discloses modifying a crystalline aluminosilicate zeolite with a fluorosilicate salt to extract surface zeolite aluminum which is replaced by silicon. U.S. Pat. No. 5,043,307 discloses modifying a crystalline aluminosilicate zeolite by steaming as synthesized zeolite containing organic template material and then contacting the zeolite in the ammonium, alkali metal, or hydrogen form with a dealuminizing agent which forms a water soluble complex with aluminum. These methods, however, often increase the complexity and operability of the process.
Therefore, it is an object of the present invention to provide a process for surface selective dealumination of crystalline aluminosilicate zeolites. It is a further object of the present invention to minimize non-selective reactions on the surface acid sites of the crystalline aluminosilicate zeolites. It is a further object of the present invention to improve process performance of crystalline aluminosilicate zeolites.